132 THE lANTHINA. 



example ; and as the Notarchus (molluscans nearly allied to 

 the Aplysise) are weaker of frame, they have, in addition to 

 this furrowed foot, a small sucker in front of it, analogous in 

 form and use to the sucker of the Nucleobranches,* which 

 they otherwise resemble in their manners. Other allied 

 Gasteropods, still more pelagic, float even on the wave ; and 

 the foot, having become useless, does either not exist, or 

 exists merely in a rudimentary condition ; while, on the 

 contrary, the pallial appendages deserve the name of fins 

 from their developement and use. Thus in the apodal Pte- 

 rosome a thin membrane surrounds the whole body, which 

 it sustains by the extent of its expansion, and displaces by 

 its muscular movements ; but in the Glaucus and Briareus, 

 the expansions are divided and extend horizontally on each 

 side. You will remember that, like the more normal spe- 

 cies, all these swim constantly in a reversed position, having 

 the foot, or ventral surface, applied against a thin layer of 

 water which intervenes between it and the atmosphere. f 



But unquestionably the most remarkable apparatus for 

 locomotion among the Gasteropods, is offered for our con- 

 templation by the lanthina, a snail-like mollusk, with a 

 light and purple-coloured shell, the native, it is believed, of 

 tropical seas, though one species has been so frequently 

 found on our own coasts, that its claim to be considered a 

 native is now generally admitted. Its proper habitat is the 

 open sea, where it has the power of swimming at the surface 

 at a slow rate. To the posterior part of the foot there is 

 attached a large vesicular appendage, very aptly named 

 spuma cartilaginea by Fabius Columna, for the vesicles are 

 as ti'ansparent as the air-bubbles in foam, while the skin 

 is cartilaginous or membranous.:}; Buoyed up on these air- 



* Rang's Man. p. 28. t Ibid. p. 24. 



X Dr. Reynell Coates gives the following description of the mode in 

 which this organ is constructed or repaired : " Individuals being placed in a 

 tumbler of brine, and a portion of the float being removed by the scissors, 

 the animal very soon commenced supplying the deficiency ; the foot was 

 advanced upon the remaining vesicles, until about two-thirds of the member 

 rose above the surface of the water ; it was then expanded to the uttermost, 

 and thrown back upon the water, like the foot of a Lymneus when commenc- 

 ing to swim ; in the next place it was contracted at the edges, and formed 

 into the shape of a hood, enclosing a globule of air, which was slowly 

 aj)plied to the extremity of the float. A vibratory movement could now be 

 perceived through the foot, and when it was again thrown back to renew 

 the process, the globule was found enclosed in its newly constructed enve- 

 lope. From this it results that the membrane which encloses the cells is 

 secreted by the foot, and that it has no attachment to the animal other than 

 the close adhesion resulting from the nice adaptation of proximate surfaces." 

 — Zool. Jown. iii. 264 ; Ann. Phil. n. s. x. 385. 



